Preparation

Optional: Laminate all photos to make the sets more durable for repeated use.

Note: Do not share the first page with students until they have matched the pairs of photographs.

Directions

Explain what glaciers are, how glaciers grow and retreat, and the two types of glaciers: continental and valley (or alpine) glaciers. Discuss the climate conditions that are necessary for a glacier to grow (snowy winters and cool summers). Discuss the climate conditions that are necessary for a glacier to shrink (warmer).

In groups of three or four, have students try to match the glacier images from the past and present. Give them approximately 10 minutes to accomplish the task. (Note: Do not share the key with students until they have matched pairs.)

Give students 5 minutes to compare their matches to those made by the other groups.

Discuss the images and reveal the correct matches.

Have students take notes about what is similar and different about pair of images.

Discussion

Discuss as a class how the "then" and "now" photos were similar and different, and hypothesize about the reason for change in glaciers.

What stayed the same? What changed?

Do all the glaciers in this sample follow the same pattern? Are they growing, retreating, or staying the same?

What climate conditions encourage glacier growth and glacier retreat?

What might account for glacier retreat today?

As glaciers get smaller, how might this affect the Earth? How might it affect humans?

Background & Extensions

Background

Glaciers are either one of two types: a continental glacier, also called an ice sheet, such as those that occur on Antarctica, or an alpine or valley glacier found in mountain valleys. The photographs in this activity are all alpine glaciers from Alaska, US. Alpine glaciers occur all over the world, yet require specific climate conditions to survive. This usually includes a location that has high snowfall in the winter and cool temperatures in the summer to prevent snow from melting.

If a glacier is to form in a given location, snow must accumulate over time, turn to ice, and begin to flow under the pressure caused by its own weight and gravity. As more and more snow accumulates over years, decades, centuries and longer periods of time, the glacier continues to move. In areas with little snowfall or low slope conditions, the glacier will flow downward and outward very slowly. If the ice is on a steep slope, if basal conditions are smooth and soft, and if there is high snowfall, then the glacier will flow faster. Often this rapid motion creates crevasses on the glacier's surface.

Glacier retreat occurs as a result of sublimation (transition of ice to vapor), snow evaporation (evaporation of liquid water in the snow), strong scouring winds, and ice melt. The process of a glacier getting smaller is called ablation. Over the past 60 to 100 years, almost all glaciers worldwide have been getting smaller and in most cases there is strong evidence that current glacier retreat is due to Earth's warming climate. This is most evident for alpine glaciers in the Arctic, which is warming quickly compared with other regions, and for alpine glaciers at high elevations in tropical latitudes.